This invention relates to a process for producing the amino acid L-serine from glycine by microbial fermentation. More particularly, it relates to the strain improvement of an L-serine producing microorganism in order to produce commercial quantities of L-serine in less time and with a minimum of contaminating amino acids.
L-serine is a non-essential amino acid which is used primarily as a dietary supplement in parenteral nutrition. Commercial production of L-serine is by fermentation of certain microorganisms which produce and accumulate L-serine in the fermentation broth. For example, U.S. Pat. No. 4,183,786 (Nakayama et al.) discloses a process for converting glycine to L-serine by use of an aqueous medium containing glycine and microbial cells of a mutant belonging to Nocardia butanica. Similarly, it is known from U.S. Pat. No. 3,623,952 (Kubota et al.) that L-serine may be accumulated by culturing certain strains of Arthrobacter citreus, Brevibacterium helvolum, Corynebacterium sp. and Candida pulcherrima on conventional media containing glycine. Additionally, U.S. Pat. No. 3,880,741 (Kageyama et al.) discloses that artificially induced mutants of Corynebacterium glycinophilum which require leucine, isoleucine, methionine, tryptophan, serine or glycine for their growth will produce L-serine from glycine in higher yields than the parent strain which does not require such nutrients.
Glycine is a costly precursor and is difficult to separate from L-serine. For these reasons, considerable research efforts have been made to improve the rate and efficiency of the bioconversion, thereby increasing L-serine product yields and decreasing levels of unconverted glycine remaining after fermentation.